Recently, much attention has been focused on GaN-based compound semiconductors (e.g., InxAlyGa1−x−yN, wherein x+y≦1, 0≦x≦1, and 0≦y≦1) for blue, green, and ultraviolet light emitting diode (LED) applications. One important reason is that GaN-based LEDs have been found to exhibit efficient light emission at room temperature.
In general, GaN-based LEDs comprise a multilayer structure in which n-type and p-type GaN-based semiconductor layers are stacked on a substrate (most commonly on a sapphire substrate with the n-type GaN-based semiconductor layer in contact with the substrate), and InxGa1−xN/GaN multiple quantum well layers are sandwiched between the p-type and n-type GaN layers. A number of methods for growing the multilayer structure are known in the art, including metalorganic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE) and hydride vapor phase epitaxy (HVPE).
In general, p-type GaN-based semiconductor layers formed by growth methods, such as MOCVD, behave like a semi-insulating or high-resistive material. This is thought to result from hydrogen passivation caused by hydrogen that is present in the reaction chamber complexing with the p-type dopant and thus preventing the dopant from behaving as an active carrier. Typically, p-type GaN-based semiconductor materials are thermally annealed to activate the p-type carriers. However, even after thermal annealing, the resistivity of p-type GaN-based semiconductor materials remains relatively high.
Since p-type GaN-based semiconductor layers have only moderate conductivity, a p-electrode typically is formed to cover substantially the entire surface of the p-type GaN-based semiconductor layer in a GaN-based light-emitting device in order to ensure uniform application of current to the entire layer and obtaining uniform light emission from the light-emitting device. However, this geometry requires that the p-electrode be light-transmissive so that light emitted by the light-emitting device can be observed through the p-electrode. Typically, the p-electrode layer must be very thin in order to be light transmissive and thus, it is difficult to attach a bonding wire directly to it. Therefore, a bonding pad is used to attach the bonding wire to the p-electrode. However, metallic materials used in the bonding pad can migrate into the p-electrode causing degradation of the light-transmission and ohmic property of the electrode. In the past, this problem has been particularly encountered when the bonding pad includes aluminum.